Induction metal detectors are generally designed to use either continuous wave signals (so-called frequency domain detectors) or to use pulse or rectangular signals (so-called time domain detectors). In a pulse induction (PI) metal detector, a coil is typically energized using a DC current, then abruptly turned off to rapidly collapse the developed magnetic field. The rapid collapse of the field at the end of each successive pulse results in eddy currents being generated in nearby conductive metallic objects. The eddy currents in turn produce their own counter-magnetic field which is detected by the metal detector's receiver.
In cases where the metal detector is being used to search for metal objects buried in the ground, the ground will often contain ferromagnetic minerals whose particles produce a decaying viscous remnant magnetic field. As the eddy currents in the metallic targets create a similarly decaying magnetic field, the two fields add together such that the coil used as a receiver for the detector has an induced emf that is the derivative of the two simultaneously decaying magnetic fields.
Needs exist for improved metal detectors.